1. Field of the Invention
The present invention relates generally to diffraction gratings for generating a plurality of beams and multi-beam pickups for recording/reproducing simultaneously to/from a plurality of tracks on an optical recording medium. The invention relates particularly to a diffraction grating having a plurality of gratings with different cycles for generating a plurality of beams and an optical pickup using such a diffraction grating.
2. Description of the Background Art
Various optics-related techniques have been researched, developed and reduced to practice in a variety of fields including communication, measurement and processing, because they permit operations at high a frequencies (high speeds), spatial information processing, phase processing and the like.
Among such techniques, the use of diffraction gratings and holograms as elements for selecting wavelengths, deflecting light, and generating a plurality of beams is known. For example, a diffraction grating in a spectroscope is used for separating a light component with a different wavelength in an incident light beam, and has a constant grating cycle.
Japanese Patent Laying-Open No. 5-264923 proposes a hologram scanner using a hologram for deflecting and collecting light as an application to a laser printer. According to this document, a light beam emitted from a semiconductor laser as an optical source is allowed to come into a plurality of scan holograms formed on a hologram disc. A light beam diffracted and deflected by the scan holograms is used to form an image on a scanning surface (photoreceptor drum), and the deflection angle of the diffracted light beam is changed by the rotation of the hologram disc for scanning. In order to correct characteristics on the scanning surface such as astigmatism and curvature of field, a grating pitch for the scan hologram is two-dimensionally modulated.
There is known such an information recording/reproducing apparatus in which an optical information recording medium (optical disc) is used, a plurality of beams are generated in an optical pickup for reading information, and the plurality of beams are directed at the optical disc at a time to obtain a signal. In the optical pickup, a diffraction grating is used to generate a plurality of beams (see Japanese Patent Laying-Open No. 1-269239 for example). Such a diffraction grating used for an optical pickup will be now described in detail.
First Conventional Example
Japanese Patent Laying-Open No. 1-269239 discloses an optical pickup including a diffraction grating for dividing a beam, a hologram element for dividing light for RES (Radial Error Signal) and FES (Focus Error Signal), and a laser source as they are integrally formed.
FIG. 1 is a view of the 3-beam optical pickup described above. Divergent light emitted from a semiconductor laser 112 as an optical source is divided into a plurality of beams by a diffraction grating 113, each transmitted through hologram element 114 as the zeroth-order diffracted light and comes into a collimator lens 115. The beams formed into parallel rays by collimator lens 115 are collected into a small enough spot on a disc 117 by an objective lens 116, and reflected as an optical beam reflecting information on disc 117. The reflected light is passed through objective lens 116 and collimator lens 115 and divided into the zeroth-order diffracted light beam and the first-order diffracted light beam by hologram element 114. The first-order diffracted light beam is allowed to come into an internal light receiving portion 118 including an RES light receiving portion, an FES light receiving portion and an RF (Radio Frequency) signal light receiving portion.
Herein, semiconductor laser 112, diffraction grating 113, hologram element 114 and internal light receiving portion 118 are built integrally as a hologram laser unit 119. Diffraction grating 113 divides a light beam into three beams, the zeroth-order diffracted light beam and the xc2x1first-order diffracted light beams, and the zeroth-order light beam is used as an RF signal and an FES, while the xc2x1first-order diffracted light beams are used as an RES. Therefore, the tracking employs the 3-beam method.
Second Conventional Example
A multi-beam optical pickup is proposed by which an optical recording medium is irradiated with a plurality of beams, which are used to record/reproduce information simultaneously to/from a plurality of tracks on the optical recording medium (Japanese Patent Laying-Open No. 1-248329).
The multi-beam optical pickup will be now described in conjunction with related figures. FIG. 2 is a view of an optical system. A light beam emitted from a semiconductor laser 102 as an optical source is formed into parallel light with a collimator lens 104, then let into a diffraction grating 103 and divided into a plurality of beams. Then, each of the beams is passed through a beam splitter 105, collected into an optical spot small enough on a disc 107 by an objective lens 106 and reflected and passed through objective lens 106 as a light beam reflecting information on disc 107. The light beam is reflected by beam splitter 105, and let into a light receiving portion 110 through a light collecting lens 108 and a cylindrical lens 109.
When a light beam is divided into for example three beams with a diffraction grating, the zeroth-order diffracted light beam (referred to as main beam) of diffraction grating 103 in the center on disc 107, and the xc2x1first-order diffracted light beams (referred to as sub beams) on opposite sides thereof are arranged in order to simultaneously perform reading with the three beams in total.
In the diffraction gratings used in the conventional pickups according to the first and second conventional examples, as shown in the plan view in FIG. 3 and the cross sectional view in FIG. 4, a grating recess 121 and a grating ridge 122 in diffraction grating substrate 130 are formed along straight lines and over an aperture surface in a constant grating cycle pn.
In the hologram scanner used for a laser printer disclosed by Japanese Patent Laying-Open No. 5-264923, the diffraction grating formed in the hologram is used for deflecting a single incident beam and does not divide this single beam into a plurality of beams. This is because this application requires a great beam intensity after diffraction, so if the beam is divided into a plurality of beams, the intensity of each diffracted light beam could be lowered, and a necessary beam intensity could not be obtained on the scanning surface. Therefore, only a diffracted light beam of the lowest order is typically used.
As a result, there is not disclosed in the document any such concept of correcting aberration in the diffracted light beam of the highest order when a plurality of high order diffracted light beams are used. The grating is formed to have curves in order to converge a beam spot in the X-direction (main scanning) and the Y-direction (sub scanning) on the scanning surface.
As described above, the diffraction grating of the optical pickup according to the first and second examples typically includes a straight line grating of an equal pitch as shown in FIGS. 3 and 4. In such a case, particularly in a multi-beam optical pickup which records/reproduces information to/from a plurality of tracks using a plurality of optical beams, the following problem is encountered.
In a multi-beam optical pickup, if the intervals Pd of beams on the disk is equal, the relation between Pd and the intervals Ph of the beams at the internal light receiving portion is given as follows:
Pd=Phxc2x7fOL/fCL
wherein the focal distance of the objective lens is fOL and the focal distance of the collimator lens is fCL.
In order to use a number of beams for recording/reproducing at a time, Pd should be small. To reduce Pd, Ph must be reduced or fCL must be increased. However, Ph is under restrictions as to the photoelectric conversion sensitivity and processing/assembling precision of the light receiving element and cannot be arbitrarily reduced, while the effective numerical aperture of the collimator lens is reduced by increasing fCL, which degrades the use efficiency of light. Therefore, the allowable image height on the disc determined by the aberration of the objective lens, collimator lens, diffraction grating, the disc and the like must be increased as much as possible.
In a conventional multi-beam optical pickup, however, a beam dividing diffraction grating having a straight line grating groove and a constant grating cycle over an aperture surface is employed, and therefore, the aberration is small for parallel light, while the aberration could be great for divergent rays as in the construction of the optical pickup disclosed by Japanese Patent Laying-Open No. 1-269239, which has been a main cause for degradation in the aberration of the entire optical system.
For example, FIG. 5 shows the wave front aberration of each beam in such a conventional multi-beam optical pickup, while FIG. 6 shows the wave front aberration derived from the diffraction grating therein. Herein, the image height refers to the position of the diffracted beam using the zeroth-order diffracted light beam on the disc as the origin.
When the reference value of each of beam wave front aberration (r. m. s. value) is 0.07xcex (Marechal Criterion), the maximum allowable image height is about 25 xcexcm. Therefore, if each of the beam intervals on the disc is 9 xcexcm, only the beams up to the xc2x1second-order diffracted beams may be available (2xc3x979=18 xcexcm less than 25 xcexcm), and therefore reading with five beams is performed. In practice, however, the number of beams is smaller when the aberration and assembling errors of the optical elements constituting the pickup are taken into account.
As a result, the wave front aberration value in the xc2x1third-order diffracted light beams exceeds the reference value as described above when the assembling tolerance of the elements is taken into account, and all the beams cannot be used for reading.
As can be seen from FIGS. 5 and 6 in comparison, a large part of the aberration of an optical system derives from the aberration of a diffraction grating.
FIG. 7 shows the wave front aberration of an optical system excluding a diffraction grating, FIG. 8 shows the wave front aberration of only the diffraction grating and FIG. 9 shows the wave front aberration of the entire optical system including the diffraction grating, each with xc2x1third-order diffracted light beams (when the image height is 0.027 mm). The wave front aberration of only the diffraction grating is in the same direction as that of the wave front aberration of the optical system excluding the diffraction grating and both aberrations are added and given in FIG. 9 as the entire wave front aberration.
As described above, in such a conventional optical pickup, straight line diffraction gratings having an equal pitch are used, the aberration for a divergent luminous flux is great, the allowable image height cannot be increased, and the number of beams cannot be increased. As a result, the spot size of the sub beams on the high image height side cannot be reduced sufficiently, which adversely affects the jitter characteristic, and reading cannot be performed at sufficiently high speeds.
As described above, conventional diffraction grating having straight line gratings of an equal pitch cannot be used for applications which require high-order beams such as a multi-beam optical pickup.
Japanese Patent Laying-Open No. 7-302435 (hereinafter as the third conventional example) discloses the use of diffraction gratings having unequal pitches as a reflective type diffraction grating for dividing a beam into three in a pickup using the 3-beam method for tracking control, which will be now described.
Herein, as shown in FIG. 10, a beam emitted from a semiconductor laser 147 has its advancing direction turned by 90xc2x0 and is divided into three beams using a reflective type diffraction grating 81 positioned at an angle of 45xc2x0 with respective to the optical axis. Each of the beams is collected on a disc 146 through a hologram element 149 and a light collecting lens 145. A light beam reflected from disc 146 is passed through light collecting lens 145, goes through the first-order diffraction (orxe2x88x92first-order diffraction) by hologram element 149 and let into a light receiving element 150.
In a pickup using a reflective type diffraction grating, since the optical axis of incident light into the diffraction grating is inclined, the distance between the zeroth-order diffracted light beam and the xc2x1first-order diffracted light beams is asymmetrical. According to the third conventional example, diffraction grating 81 is formed to have a cycle longer along the direction of the arrow X as shown in FIG. 11 in order to correct this asymmetry.
More specifically, diffraction grating 81 according to the third conventional example has unequal pitches and this is for the purpose of correcting the diffracted light beam so that the +first-order diffracted light beam and xe2x88x92first-order diffracted light beam will be symmetric with respect to the zeroth-order diffracted light beam, rather than correcting the aberration of the xc2x1second or higher order diffracted light beams, and if the xc2x1first-order diffracted light beams satisfy the conditions of a defined wave front aberration, it does not mean that beams of higher orders also satisfy the conditions of the wave front aberration. According to the third conventional example, a method of designing to satisfy a prescribed wave front aberration for beams of xc2x1second or higher orders is not suggested.
In the multi-beam optical pickup described above, the wave front aberration of higher order diffracted light beams should also be equal to or lower the defined level, and therefore the diffraction grating according to the third conventional example cannot be applied.
In addition, the third conventional example is directed to a solution to the asymmetry of the xc2x1first-order diffracted light beams unique to the reflective diffraction grating, and a transparent type diffraction grating having the xc2x1first-order diffracted light beams being symmetric is not addressed by the third conventional example.
As a result of research by the inventors, however, it was found that the wave front aberration of a higher order diffracted light beam was particularly disadvantageous when a divergent luminous flux was used in a transparent diffraction grating as shown in FIGS. 5 and 6.
Furthermore, although the grating according to the third conventional example is formed by straight lines, such straight line grating can only reduce the wave front aberration to a limited level.
It is an object of the present invention to provide a diffraction grating for generating a plurality of beams which can restrain the front wave aberration of higher order sub beams as well as a main beam.
Another object of the present invention is to provide a diffraction grating for generating a plurality of beams which permits the spot size of each beam to be sufficiently limited and the jitter characteristic to be improved.
Yet another object of the present invention is to provide a multi-beam optical pickup which can restrain the wave front aberration of higher order sub beams as well as a main beam.
A still further object of the present invention is to provide a multi-beam optical pickup which permits the spot size of each beam to be sufficiently limited and the jitter characteristic to be improved.
According to one aspect of the present invention, the diffraction grating for generating a plurality of beams includes a plurality of gratings formed to have an axis of symmetry perpendicular to the direction in which a beam is divided on the surface of the diffraction grating and includes at least two gratings having different fundamental cycles.
The diffraction grating for generating a plurality of beams includes a plurality of gratings on the surface of thereof formed to have an axis of symmetry perpendicular to the direction in which a beam is divided, and at least two of the plurality of gratings have different fundamental cycles, so that the aberration in the periphery of divergent light may be reduced.
According to another aspect of the present invention, the diffraction grating for generating a plurality of beams includes a plurality of gratings, which are formed in a curved shape on the surface of the diffraction grating and include at least two gratings having different fundamental cycles.
The diffraction grating have a plurality of gratings formed in a curved shape on the surface thereof and at least two of the plurality of gratings have different fundamental cycles, so that the aberration in the periphery of divergent light may be reduced.
According to a further aspect of the present invention, a multi-beam optical pickup includes a semiconductor laser, a diffraction grating for generating a plurality of beams having a plurality of gratings which have an axis of symmetry perpendicular to the direction in which a beam is divided on the surface and at least two of which have different fundamental cycles, a hologram element for transmitting a light beam transmitted through the diffraction grating for generating a plurality of beams and reflected from a recording medium, and an internal light receiving portion for receiving a light beam transmitted through the hologram element.
The diffraction grating for generating a plurality of beams have a plurality of gratings which have an axis of symmetry perpendicular to the direction in which a beam is divided on the surface thereof, and at least two of which have different fundamental cycles, and therefore the wave front aberration of the diffraction grating for generating a plurality of beams may be reduced, so that the wave front aberration of the entire optical system including the diffraction grating for generating a plurality of beams may be reduced.
According to an additional aspect of the present invention, a multi-optical pickup includes a semiconductor laser, a diffraction grating for generating a plurality of beams having a plurality of gratings which are formed in a curved shape on the surface and at least two of which have different fundamental cycles, a hologram element for transmitting a light beam transmitted through the diffraction grating for generating a plurality of beams and reflected from a recording medium, and a internal light receiving element for receiving a light beam transmitted through the hologram element.
The diffraction grating have a plurality of gratings which are formed in a curved shape on the surface and at least two of which have different fundamental cycles, and therefore a wave front aberration in the opposite direction to the wave front aberration of the optical system excluding the diffraction grating for generating a plurality of beams may be generated in order to reduce the wave front aberration of the entire optical system including the diffraction grating for generating a plurality of beams.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.